1 Field of the Invention
The present invention relates to a desulfurization apparatus for coal gas containing sulfur, and an electric power plant using the same.
2 Description of the Related Art
Among fossil fuels, coal is distributed widely in the world in comparison with oil and natural gas. Fossil fuel reserves are plentiful and are expected to be used in the future for generating electric power. A known method of generating electric power involves pulverizing coal in order to be burned.
In view of thermal efficiency and adaptability to the environment, there has been developed a coal gasification compound electric power plant in which coal is gasified. The gasified coal is first desulfurized, burned, and then the burned gas is then fed to a gas turbine and a steam turbine to generate electric power.
FIG. 1 is a block diagram of a conventional coal gasification combined electric power plant using a desulfurization apparatus. Electric power plant 21 has coal gasification equipment 22, gas purification equipment 41, and compound electric power generating equipment 24. The coal gasification equipment 22 includes gasification furnaces 27a and 27b, that mix pulverized coal 25 and a gasification agent 26 (normally, oxygen) and perform gasification under predetermined conditions. A gas cooler 28 cools coal gas 11 which is exhausted from the gasification furnace 27b.
The coal gas 11 exhausted from the gas cooler 28 passes through the desulfurization tower 2 within the dry desulfurization apparatus 1. Thereafter, the gas 11 passes through a filter 29 provided within the scrubbing apparatus 23. Then, the gas 11 is supplied to the combined electric power generating equipment 24.
The coal gas 11 is burned in a burner 30 provided within the combined electric power generating equipment 24. The burned gas is supplied to an exhaust heat recovery boiler 33 through a gas turbine 31. A condenser 36 and a steam turbine 35 are provided in the exhaust heat recovery boiler 33.
Operation of the coal gasification combined electric power plant 21 having the above structure will be described below. The pulverized coal 25 and the gasification agent 26 are mixed. The mixed gas is supplied to the gasification furnace 27a which has a high temperature. In the gasification furnace 27a, a reaction occurs in which carbon is mainly oxidized to carbon dioxide. An inner portion of the gasification furnace 27b is under high pressure and a reduction reaction takes place mainly between the carbon dioxide and carbon therein. Carbon monoxide is produced through the reduction reaction. Accordingly, the gasification furnaces 27a and 27b gasify the coal at a high temperature and a high pressure (which varies according to the gasification method, for example, about 1400.degree. C. and about 2 MPa) so as to produce the coal gas 11. The produced coal gas 11 is composed of carbon monoxide, hydrogen, carbon dioxide, and water vapor.
The coal gas 11 is cooled to a suitable temperature (about 500.degree. C.) in the gas cooler 28. The cooled coal gas 11 is fed to the desulfurization tower 2 within the gas purification equipment 41.
The desulfurization tower 2 removes H.sub.2 S contained in the coal gas 11. The coal gas 11 exhausted from the desulfurization tower 2 passes through the filter 29 thereafter. The filter 29 removes dust contained in the coal gas 11. Accordingly, the desulfurization tower 2 and the filter 29 remove sulfur and fine particles that cause corrosion and abrasion of the gas turbine by passing the coal gas 11 therethrough.
The clean coal gas 11 from which sulfur and fine particles are removed is supplied to the combined electric power generating equipment 24.
The clean coal gas 11, purified in the gas purification equipment 41, is burned in the burner 30. The burned combustion gas rotates the gas turbine 31 to generate electric power. Exhaust gas 32 is exhausted from the gas turbine 31 and fed to the exhaust heat recovery boiler 33. The exhaust heat recovery boiler 33 takes the heat from the exhaust gas 32 so as to produce steam 34. Steam 34 rotates the steam turbine 35 to generate electric power. The steam 34, exhausted from the steam turbine 35, is condensed in the condenser 36. A part of the condensed steam is fed back to the exhaust heat recovery boiler 33. The remainder of the condensed steam is fed to the gas cooler 28.
Further, heat recovered from the coal gas 11 in the gas cooler 28 is combined with the steam 34 fed from the exhaust heat recovery boiler 33 fed to the steam turbine 35. Thereafter, the steam 34 is discharged from the steam turbine 35 and returned to the gas cooler 28 through the condenser 36.
When coal is gasified, most of the sulfur contained in the coal becomes hydrogen sulfide and becomes mixed with the coal gas 11. A convention desulfurization apparatus for removing the hydrogen sulfide at a high temperature using iron oxide as the desulfurization agent, is the dry desulfurization apparatus 1. The dry desulfurization apparatus 1 performs desulfurization while keeping the temperature of the coal gas 11 high for as long as possible. The fluid upon which the desulfurization function has been performed is supplied to the compound electric power generating equipment 24. The dry desulfurization method exhibits excellent thermal efficiency.
The structure of the dry desulfurization apparatus 1 will be described below with reference to FIG. 2. It comprises a desulfurization tower 2, a regeneration tower 3, a reduction tower 37, a sulfur condenser 8, a circulation gas compressor 9 and a heater 10.
The coal gas 11 flows from an end (the lower portion in the drawing) of the desulfurization tower 2. The flowing coal gas 11 is mixed with desulfurization agent 38 provided within the desulfurization tower 2. A chemical reaction between the coal gas 11 and the desulfurization agent 38 occurs according to formula (1). EQU Fe.sub.2 O.sub.3 +2H.sub.2 S+H.sub.2.fwdarw.2FeS+3H.sub.2 O (1)
The coal gas 11 from which the sulfur is removed thereafter flows out from the other end (the upper portion in the drawing) of the desulfurization tower 2.
The desulfurization agent 38 which absorbs the sulfur contained in the coal gas 11, thus becoming a sulfide, and is fed to an end (the upper end in the drawing) of the regeneration tower 3. Air 39, including oxygen, is supplied from the other end (the lower portion in the drawing) of the regeneration tower 3. A chemical reaction shown in formula (2) occurs between the sulfide and the air within the regeneration tower 3, so that the sulfide is oxidized. The sulfide can be regenerated by this oxidizing reaction. EQU 4FeS+7O.sub.2.fwdarw.2Fe.sub.2 O.sub.3 +4SO.sub.2 (2)
The regenerated desulfurization agent 38 is again fed to the desulfurization tower 2 and reused. The desulfurization agent 38 is moved between the desulfurization tower 2 and the regeneration tower 3 by air current transmission.
The sulfur removed from the desulfurization agent 38 becomes a sulfurous acid gas in the regeneration tower 3. That gas is then fed to the reduction tower 37 as a regeneration tower outlet gas 14. The regeneration tower outlet gas 14 is supplied to an end (the lower portion in the drawing) of the reduction tower 37 and undergoes a chemical reaction (3) with a smokeless coal 40 supplied to the other end (the upper portion in the drawing) of the reduction tower 37, as shown in the following formula: EQU 2C+2SO.sub.2.fwdarw.2CO.sub.2 +S.sub.2 (3)
A sulfur steam is produced by the chemical reaction that flows into the sulfur condenser 8 (the upper portion in the drawing).
The sulfur steam is cooled within the sulfur condenser 8. The sulfur steam is condensed by the cooling and discharged to an outer portion of the dry desulfurization apparatus 1 as the chemical element sulfur 16.
The tail gas 17 that flows from the other end (the lower portion in the drawing) of the sulfur condenser 8 is fed to the circulation compressor 9 and the pressure thereof increases. The tail gas 17 discharged from the circulation compressor 9 is fed to the heater 10 and the temperature thereof increases. A portion of the heated tail gas 17 is mixed with the coal gas 11 supplied to the desulfurization tower 2 and is then fed to the desulfurization tower 2. Some of the tail gas 17 is mixed with air 39 for regeneration and is then fed to the regeneration tower 3.
In the conventional desulfurization apparatus described above and the electric power plant using the same, the following problem has occurred. Since the desulfulrization reaction in tower 2 and the regeneration reaction in regeneration tower 3 involve solid-to-gas reactions, the overall reaction speed is less than that of a wet desulfurization apparatus. Furthermore, the reaction rate of each of the chemical reactions is low. Accordingly, the amount of desulfurization agent 38 required in the desulfurization tower 2 and the regeneration tower 3 is large and so tower 2 and regeneration tower 3 must be large in comparison to those required in a wet desulfurization apparatus. As a result, the dry process is costly.
Further, as the amount of desulfurization agent 38 increases and the power required for the air current transmission equipment of the desulfurization agent 38 and the electric power plant increases, the total size of the compound electric power plant using the conventional dry desulfurization apparatus increases. In addition, the power consumed within the electric power plant increases.